She would have done anything she could to help Hitler win the war: Prof Christopher Andrew.

As early as 1934, both the Security Service and the Foreign Intelligence Service (MI6), were aware of her foreign trips.

"That's quite significant,” according to Professor Christopher Andrew, official historian of MI5. "These were very small organisations at the time and if they targeted someone it meant that they regarded him or her as a real priority.”

MI5 learned of Diana's secret marriage to Mosley, in Germany, in Hitler's presence, soon after it had taken place in 1936. The wedding was not formally announced for another two years.

They were aware of her regular trips to Germany where, travelling as Mrs Bryan Guinness, she acted as a liaison between Mosley and Hitler.

Once war broke out, the Home Office initially decided to keep Lady Mosley at large, but after a flurry of letters from her extended family, they interned her at Holloway jail.

Her former father in law, Lord Moyne, wrote of her "extremely dangerous character". The sister of Mosley's first wife, Baroness Ravensdale, told the Home Secretary himself that she was a "real danger". Her source was her young niece, Mosley's daughter. "My niece tells me Diana Mosley goes up every day to run the British Union Office,” she stated. “You may have incarcerated the leader but with this going on … the door still seems to me to be open.”

Anne De Courcy, author of a new biography of Diana Mosley, believes that many of Diana's extended family bore grudges against her. She doesn't believe Diana was a real security risk and believes it was cruel of the Home Office to separate Diana from her baby sons (the second, Max, was just 11 weeks old when she was interned in 1940).

"There was this young woman, breastfeeding an 11 week old baby every four hours … even if she'd wanted to she couldn't have been a particularly valuable conduit passing information to the enemy during the war. So yes I think that was harsh."

But Professor Andrew agrees with MI5's assessment. Diana Mosley dazzled almost every man she met - she was described as more beautiful than Botticelli's Venus - yet he believes she would have done anything she could to help Hitler win the war.